Graphene, the one-atom thick, two-dimensional carbon allotrope, has received significant attention owing to its extraordinary properties which, among several potential outlooks, hold promises to revolutionize coating applications. Yet, the full technological potential of graphene coatings still requires better understanding of how the atomic monolayer alters the wetting properties of the underlying substrate. From a more fundamental viewpoint, this raises the issue of the respective roles of short- and long-range interactions in the wetting properties of a substrate by a liquid.
To this date, the topic remains highly controversial because of the challenge of defining an unambiguous experimental system able to discriminate the intrinsic wettability from the influence of ambient adsorbates. In this paper, we suspend graphene monolayers over silicon surfaces bearing nanopillars. The precise control over the nanopillar profile allows us to vary the area fraction of suspended graphene from 0% to 95% which is used to effectively “tune” the liquid-substrate interactions. Our system further allows to introduce water in the free space between the nanopillars, thus reaching the uncommon situation where water drops are deposited on graphene as it virtually floats over water.
Our approach leads to two main results.
- First, fully suspended graphene exhibits the highest water contact angle (85°±5) compared to partially suspended and supported graphene, regardless of the hydrophobicity (hydrophilicity) of the underlying substrate.
- Second, in the presence of a substrate, we could quantify that ca. 80% of the long-range water-substrate interactions are screened by the graphene monolayer.
This study shows that graphene opens new ways to engineer surface energy of surfaces and to investigate the relationship between molecular interactions and macroscopic wetting properties, an issue which remains largely unanswered in wetting science.
This work was initiated during the fellowship of Antonio Checco in CEMES funded by the Laboratory of Excellence NEXT and was supported, in part, by the project SUGAREE funded by the Laboratory of Excellence NEXT n° ANR-10-LABX-0037.
Wettability of partially suspended graphene
T. Ondarçuhu, V. Thomas, M. Nuñez, E. Dujardin, A. Rahman, C. Black, A. Checco, Sci. Rep., 6 (2016) 24237 http://dx.doi.org/10.1038/srep24237.
Dr. Thierry Ondarcuhu : ondar chez cemes.fr